As the rapid development of the information technologies, the information communication of voice, images and data is increasing. The most of all, as the extensive use of the internet, people raise higher request to the wide-band communication. In order to meet the needs of wide-band communication by low-cost and high-quality system in a shortest possible time, wavelength division multiplexing (WDM) technology is widely used. WDM technology increases the number of channels basing on the principle of different wavelengths can be transmitted in a same path. This improves the signal transmission multiplied. The capacity of the fiber-optical communication can be expanded without setting up new path of optical fibers by WDM so that the cost of the network construction is reduced.
Another way of improving the transmission capacity of the optical fiber is to further reduce channel spacing. At present, the channel spacing specified by International Telecommunication Union (ITU), is 100 Hz or 200 Hz. The optical interleaver is preferred for the expanding of the capacity of the current system with lower cost because the optical interleaver can change the channel spacing without changing of the current equipments and system. A 100 Hz optical interleaver can be used for the current system with channel spacing of 200 Hz and a 50 Hz optical interleaver can be used for the current system with channel-spacing of 100 Hz. The function of the optical interleaver is to split an equally spaced optical signal into an odd channel and an even channel alternately by expanding the channel spacing. The channel spacing is doubled which is called demultiplexing. Or the optical interleaver multiplex two signals into one signal by reducing the channel spacing. The optical interleaver can expand the capacity without changing of the current equipments and thus the cost is reduced. So the optical interleaver is very important for the upgrade of the existing fiber-optical communication WDM systems. At present, the commonly used optical interleavers are fiber Bragg Grating type, birefringent crystal type, Mach-Zehnder interferometer type, and resonant cavity type.
In the existing technology, the optical interleavers with birefringent crystal are compact structure with reflecting mirror. U.S. Pat. No. 6,697,198 disclosed a crystal type of optical interleaver with reflecting structure. As shown in FIG. 1, the first optical signal beam entering from the first port 400 passes through the first optical path D1 and splits into two beams by the first walk-off crystal 415, i.e., an upper light beam and an under light beam. Then the first optical signal beam passes through an interference crystal group 418 and is splitted into a second optical signal beam and a third optical signal beam by the second walk-off crystal 430. The second optical signal beam and the third optical signal beam are reflected to the second walk-off crystal 430 by the reflector 450. Then the second optical signal beam and the third optical signal beam pass through the second optical path D2 and the third optical path D3 respectively. After passing through the interference crystal group 418, the second optical signal beam and the third optical signal beam output via the collimator 495 of the second port and the collimator 490 of the third port. The first optical signal beam passes through the first optical path and is split into a second optical signal beam and a third optical signal beam by the second walk-off crystal 430. The second optical signal beam and the third optical signal beam pass through the second optical path and the third optical path respectively. The second optical path and the third optical path are coupled with the first optical path and output through the collimator 495 and the collimator 490 after passing through the interference crystal group 418. The interference crystal group 418 includes six light beams of the first optical path, the second optical path and the third optical path. So the interference crystal group 418 needs to have a larger volume to accommodate the six light beams and the cost of the device is increased.